(a) Field of the Invention
The present invention relates to an automatic transmission for vehicles, and more particularly, to a shift control method for an automatic transmission that can reduce shift shock when shifting from a reverse R range to a drive D range.
(b) Description of the Related Art
In the automatic transmission used for vehicles, a shift control system performs control to realize automatic shifting into different speeds and shift ranges according to various factors including throttle opening, vehicle speed and load, and several engine and driving conditions detected through a plurality of sensors. That is, based on such factors, the shift control system controls a plurality of solenoid valves of a hydraulic control system such that hydraulic flow in the hydraulic control system is controlled, resulting in the shifting of the transmission into the various speeds and shift ranges.
Particularly, when a driver shifts a selector lever from a reverse R position to a drive D position, shift shock occurs by inertia force of the vehicle and a clutch control problem.
That is, when the shift selector lever is shifted from the reverse R position to the drive D position during a reverse driving state, forward clutches are engaged in a state where the vehicle is moving in a reverse direction, resulting in shift shock.
Therefore, the present invention has been made in an effort to solve the above problems.
It is an objective of the present invention to provide a shift control method for an automatic transmission that can prevent shift shock when shifting from a reverse R range to a drive D range by slip-controlling forward clutches.
To achieve the objective, the present invention provides a shift control method for an automatic transmission comprising:
determining if a signal for shifting from a reverse range to a drive range is input;
determining if an engine is in an idle state when the signal for shifting from the reverse range to the drive range is input;
outputting an initial shifting signal of a first duty ratio to a drive unit and maintaining the first duty ratio for a first predetermined fill time when the engine is in the idle state;
reducing, when the first predetermined fill time is elapsed, the first duty ratio to a second duty ratio and outputting a signal of the second duty ratio for a soft engagement control to the drive unit;
determining if a current turbine rpm is less than a first predetermined value which is obtained by extracting a second predetermined value from a target turbine rpm;
outputting, when the current turbine rpm is less than the first predetermined value, a feedback duty control signal to the drive unit;
determining if a clutch is in a slip engagement state;
ending, when the clutch is in th slip engagement state, the feedback duty control, and after correcting the first duty ratio, outputting an open loop duty control signal to the drive unit;
determining if an engine is out of an idle state;
determining, when the engine is out of the idle state, if a shifting synchronization is completed;
correcting, when the shifting synchronization is completed, the throttle opening and outputting a duty control signal for increasing the duty ratio to a third predetermined value and maintaining the increased duty ratio for a predetermined duration to the drive unit.
The shift control method may further comprise the steps of outputting a current shifting signal when the engine is not in the idle state, and ending the duty control.
The shift control method may further comprise the steps of determining, when the clutch is not in the slip engagment state, if a vehicle speed is equal to a fourth predetermined value, and outputting, when the vehicle speed is equal to the fourth predetermined value, a duty control signal for increasing a duty ratio to a fifth predetermined value and maintaining the increased duty ratio for a predetermined duration to the drive unit.